prolog-examples

This repository includes some code samples to get started with list manipulation in Prolog.

Setup

To run Prolog code, you first need to install SWI Prolog on your machine. The stable version is available here. Once installed, run swipl in a Linux terminal or open the SWI-Prolog application on Mac OS. You will see a prompt ?- appear in the terminal.

How do I run the code samples?

To run the code samples from a Prolog prompt, simply use the following:

consult('filename.pl').

If you need to change directories, simply type:

cd('/path/to/your/dir').

Description of the code samples included in this repository

Testing if a list has an even number of elements: listevenelements.pl

This file includes two predicates. The first one listlength computes the length of a list using an accumulator. The second one checks if the list has an even number of elements. To test the predicate, you can use the following:

?- even([a,b,c,d]).
true.

?- even([a,b]).
true.

?- even([]).
true.

?- even([a,b,c]).
false.

?- even([a,b,c,d,e]).
false.

Testing if a list has two elements: listtwoelements.pl

This file includes two predicates. The first one is the same listlength than in the previous example file. The second one two checks if a list has exactly two elements. To try out the predicate two, some examples are:

?- two([a,b]).
true.

?- two([a,b,c]).
false.

?- two([]).
false.

Testing if a list is a set: set.pl

This file includes two predicates. The predicate isnotinlist tests if the first argument is in the list passed as the second argument. The predicate set tests if the list is a set (i.e. no elements are included more than once in the list). To test the predicate set, you can use the following examples:

?- set([a,b,c]).
true .

?- set([a,[b,a],c]).
true .

?- set([]).
true.

?- set([a,b,c,c,d]).
false.

Note that the predicates are not the optimal way to implement this test. The check is done in O(N^2). Instead, one could use predefined predicates from SWI-Prolog like setof and length to implement a O(nlogn) solution.

Testing if a list is a subset: subset.pl

This file includes two predicates isinset and issubset that respectively test if an element is in a set and if a set is a subset of a second set. To try out this sample, here are a few examples:

?- issubset([a,b],[a,b,c,d]).
true .

?- issubset([e,f],[a,b,c,d]).
false.

?- issubset([b,a],[a,b,c,d]).
true .

?- issubset([a,b],[a,c,d,b]).
true .

?- issubset([],[a,b]).
true.

Contact Info

Ways to get in touch with me:

• Twitter: https://www.twitter.com/nicolaspapernot
• Webpage: https://www.papernot.fr